The differential decay rate for 
is given by
The Dalitz plot density
is a Lorentz invariant function which contains 
in the
following form,
with
The quantities 
are related to the ones quoted by the PDG [3]
by
To obtain the rate for 
, one replaces in (3.17)
by 
.
For convenience we also display the 
rates evaluated in the
approximation (3.8) for the form factors,
We have used the physical masses [3] in evaluating these ratios and
to scale the slope in both cases. The
terms linear and quadratic in 
are proportional to 
and
therefore strongly suppressed in the
electron case. We do not include them in the denominators,
because these coefficients
are smaller than 
. The interference term 
is
absent by angular momentum conservation.
Furthermore, one has
